1. Field of the Invention
The subject invention relates to a viscoelastic polyurethane foam having a density of from one to twenty pounds per cubic foot. More specifically, the subject invention relates to the viscoelastic polyurethane foam being formed of a composition having a chain extender that improves physical properties and viscoelasticity of the foam.
2. Description of the Related Art
Various related art viscoelastic foams are a reaction product of an isocyanate component and an isocyanate-reactive component reactive with the isocyanate component. These related art foams are illustrated in U.S. Pat. No. 6,204,300; European Patent Application No. 1,178,061; and PCT Publication WO 01/32736.
Viscoelastic polyurethane foam is currently a niche application in the United States. It is used mainly in home and office furnishings, although a considerable amount of work has been conducted for automotive applications. Certain automotive applications subject the viscoelastic foam to a wide range of temperatures, especially in areas that have very cold seasons and very hot seasons. Therefore, the foam has to be able to perform satisfactorily at both the colder and warmer temperatures. The market for viscoelastic foam in home furnishing applications is currently estimated at about 25 million lbs./yr. in the United States. While the market size is now relatively small, it is growing at an estimated rate of about 20% to 30% per year.
Viscoelastic foam exhibits slow recovery, and thus high hysteresis, during a compression cycle. They also typically have low ball rebound values. These properties may result from either low airflow, as the recovery is limited by the rate of air re-entering the foam, or by the inherent properties of the foamed polymer. Polymer viscoelasticity is usually temperature-sensitive, and is maximized when the polymer undergoes a glass transition. For the viscoelastic foams currently studied, this glass transition results from vitrification of the polyether soft segment phase. By manipulating the structure and composition of the soft segment phase so that the glass transition temperature approximately coincides with a “use temperature” of the material, the viscoelastic nature of the material is maximized. When this material is used in a mattress or as a seat cushion, body heat from the user warms a portion of the material, thus softening it. The result is that the cushion molds to the shape of the body part in contact with it, creating a more uniform pressure distribution, which increases comfort. In addition, the remainder of the material remains hard, providing support. Thus, the temperature sensitivity increases the effective support factor of the material, allowing the construction of a mattress without metal springs.
The type of isocyanate component and the functionality and hydroxyl value of the isocyanate-reactive component are selected and formulated such that the glass transition occurs at a temperature at which the foam is used. While most of the physical properties of viscoelastic foams resemble those of conventional foams, the resilience of viscoelastic foams is much lower, generally less than about 15%. Suitable applications for viscoelastic foam take advantage of its shape conforming, energy attenuating, and sound damping characteristics. One way to achieve these characteristics is to modify the amount and type of isocyanate-reactive components, isocyanate components, surfactants, catalysts, fillers, or other components, to arrive at foams having low resilience, good softness, and the right processing characteristics. Too often, however, the window for processing these formulations is undesirably narrow. These approaches are shown in U.S. Pat. Nos. 6,495,611; 5,420,170; and 4,367,259. Other related art foams are shown in U.S. Pat. Nos. 4,334,031; 4,374,935; and 4,568,702; PCT Publication WO 01/25305; European Patent No. 0934962; and European Patent Application No. 1125958 and 0778301. However, none of these related art patents discloses or suggests the unique and novel polyurethane viscoelastic foam of the subject invention.
Other approaches to making viscoelastic foam hinge on finding the right mixture of polyether polyols and other components. For example, U.S. Pat. No. 4,987,156 arrives at a soft, low-resilience foam with a mixture of high and low molecular weight polyols, each of which has a functionality of at least 2, and a plasticizer having a solidification point less than −20 degrees C. However, the '156 patent does not disclose a viscoelastic foam and requires that the polyol and the isocyanate be reacted in the presence of the plasticizer. U.S. Pat. No. 5,420,170 teaches use of a mixture that includes one polyol having a functionality of 2.3–2.8 and another polyol having a functionality of 2–3. U.S. Pat. No. 5,919,395 takes a similar approach with a polyol mixture that contains a 2200 to 6200 weight-average molecular weight polyol having a functionality of 2.5 to 6 and a rigid polyol having molecular weight 300 to 1000 and a functionality of 2.5 to 6. Neither the '170 patent nor the '395 patent disclose adding a chain extender to the composition to modify the glass transition temperature of the foams.
Another related art composition is disclosed in a paper titled “Novel MDI-Based Slabstock Foam Technology” by Lutter and Mente. The composition disclosed produces a viscoelastic foam from an isocyanate-terminated prepolymer, a first polyol, and an ethylene-oxide rich polyol. However, the paper does not disclose a chain extender present in significant amounts to produce the viscoelastic foam having improved properties.
The related art foams described above are characterized by one or more inadequacy. Accordingly, it would be advantageous to provide a viscoelastic polyurethane foam that overcomes these inadequacies.